Studies were performed to investigate the cellular and molecular events associated with the activation of mononuclear phagocytes. We have characterized the expression of the three known subunits of the interleukin-2 (IL-2) receptor on human monocytes. It was found that such subunits are independently regulated by exogenous signals and utilize different molecular mechanisms to modulate their expression. Studies were also performed on key enzymes controlling the amino acid metabolism in murine macrophages and human monocytes. These studies have led to the discovery that the catabolism of tryptophan and that of arginine are connected and mutually regulated by degradation products. These results are of major relevance because the catabolites of such amino acids are biologically very relevant and thus potential targets of clinical applications.